Required reading: these are titles that are not only something anyone can enjoy regardless of genre preference, but they're also classics or titles that will soon enough become classics that should be on anyone's bookshelf.
Highly recommended: these are titles that are highly recommended, but for various reasons are not for everyone.
If only...: as you might guess, these are books that would be SO much better "if only..." *something* was changed. Not highly recommended, but not necessarily "meh" material just yet.
Meh: I'll use this whenever a title doesn't do it for me,
Just say no!: this will hopefully not be used often, but is basically the most negative I can get regarding a book without getting nasty!